The present invention is directed towards pitch based graphite fabric or felts made from stretch broken pitch precursor yarns for use in fuel cell. gas diffusion layer substrates and high thermal conductivity reinforced composites and the like.
The use of carbonaceous material in conjunction with electron collection is well known. The function of the carbon or graphite has primarily been that of an electrical current (a currency) collector. A number of carbonaceous fiber based substrates have been proposed for fabricating gas diffusion layers (xe2x80x9cGDLsxe2x80x9d) in fuel cell and forming specialized reinforced plastic composites. In a first application, the carbon or graphite fibers are used to create a porous substrate exhibiting a good electrical conductivity. In a second application the fiber is used to provide high mechanical properties and if desired raise the thermal conductivity of the reinforced plastic. High in-plane and through-the-thickness thermal conductivity reinforced plastic mounting plates are desirable, for example, in electronic applications where a large amount of heat needs to be rapidly dissipated away from electronic components mounted on the plates.
Fuel cell GDLs have been fabricated from papers, felts and fabrics using a number of polyacrylonitrile (xe2x80x9cPANxe2x80x9d) derived fibers. Fuel cells and other electrochemical devices are typically built from an assembly of bipolar plates, a GDL, a catalyst layer and a membrane. Such a device is shown in FIG. 1. The gas diffusion layer is also referred as membrane electrode or electrode substrate.
The fibrous GDL substrate is generally coated on one side or both sides with a carbonaceous mixture, the mixture containing fine graphite powders and various conductive fillers. A catalyst may be deposited within the porosity or at the surface of the coating.
While the GDL substrate is frequently fabricated with a PAN based paper, PAN based woven fabric or needled felt can be used. It is believed that the latter forms provide better handling ability as they have higher tensile strength than a paper media. These characteristics are essential in carrying the fibrous support during the coating operations. Several references refer to the use of PAN fiber to fabricate the GDL media. In particular, PCT Publication No.: WO 01/04980 describes the use of a low cost PAN to fabricate various forms of GDL media. In applications involving fuel cells, it is desirable that the gas diffusion layer so formed be as thin as possible. Accordingly, the fabric used in such application should be thin and have a smooth surface.
Typically, in fuel cell design, the base fabric is created by spinning yarns from staple PAN filament that typically ranges in length from one to two inches. These yarns are then woven into a plain weave fabric. The woven fabric is then carbonized by a heat treatment process in a nitrogen atmosphere. The now carbonized fabric is subject to a further heat treatment (at a higher temperature) to graphitize it, also in a nitrogen atmosphere. The fabric is subsequently coated with a carbonaceous mixture on which a platinum based catalyst may be deposited. Some fuel cell stack fabricators elect to apply the catalyst on the membrane.
PAN based fibers are the lowest cost carbon or graphite fibers available on the market. However, PAN fibers exhibit fairly poor electrical and thermal properties when compared with pitch based carbon or graphite fibers. Pitch derived carbon or graphite fibers exhibit electrical conductivity four to six times greater than PAN derived fibers and are a better choice than PAN fibers in a fuel cell application where superior electro-conductivity is needed to enhance overall fuel cell performance. An object of the present invention is to overcome the drawbacks of the existing forms and high cost of pitch fibers. Pitch fibers are available in costly large tow yarns or in the form of chopped fibers. None of these forms are suitable for fabricating a thin flat fabric or needle felt. The smallest denier commercially available in pitch is a tow of 3850 denier, which would generate a heavy thick GDL layer. Another limitation of typical commercial pitch fiber is their high moduli which limits their forming ability. For example, it is impossible to needle punch a highly carbonized or graphitized pitch fiber. One approach to yield a suitable size yarn for weaving or a suitable web for needling a felt is to subject tows of pitch fiber in a thermoset state to a stretch breaking process.
Reinforced plastics used for heat dissipation can also benefit from the invention. In such applications, mounting plates supporting electronic components play a structural role and act as conduits to dissipate heat away from electronic components. Pitch fibers, in the form of unidirectional fiber lay-up, sheet molding compound, paper and fabrics, are already used in these applications. The textile forms derived from the invention will help provide the electronics industry with lower cost thin fabric or needled punch felt that exhibits high through-the-thickness thermal conductivity. Following graphitization of the thermoset pitch textile, plates or other geometries may be readily fabricated into a rigid component through densification with thermoset or thermoplastic polymers.
It is therefore a principal object of the invention to provide for the use of pitch precursor graphite fibers in unique forms in increased applications, including fuel cells and in high thermal conductivity reinforced composites.
It is a further object of the invention to provide for the use of pitch precursor graphite fibers in unique forms, which may be woven into relatively thin fabrics or needle punched in thin mats.
It is a yet further object of the invention to provide for such fiber forms which are relatively inexpensive.
A further object of the invention is to provide for a fabric or a mat made from pitch precursor graphite fiber in unique forms having superior thermal and electrical conductivity.
A further object of the invention is to provide for a fabric or a mat made from a blend of pitch precursor graphite fiber in unique forms and PAN based graphite fiber.
These and other objects and advantages are provided by the present invention. In this regard the present invention takes pitch precursor yarn at the thermoset stage, which is prior to carbonization or graphitization. This yarn is relatively thick, i.e. 3850 denier or more. The yarn is then stretch broken by stretch breaking. Stretch breaking involves a process that starts with higher denier yarns and reduces them to lower denier yarns whereby the multiple filaments within the yarn bundle, are randomly broken and then drawn to a lower denier. These are then recombined in a durable yarn or in the form of a web also called a ribbon. The yarn is then woven or otherwise formed into a thin fabric, which is subject to heat treatments to convert the yarns into highly graphite yarns. Alternatively, the web can be stacked to a given thickness and at the desirable fiber orientation and needle punched. These yarns have the same relative properties that are obtained by the more expensive process of heat treating yarns and then forming a fabric therefrom. The fabric or the mat can be used in a fuel cell by impregnating or coating it with an appropriate carbonaceous mixture or used to fabricate high thermal conductivity reinforced plastic composites.